This invention relates to methods, apparatus and systems related to the fault diagnosis, testing and management of optical networks, particularly in the distribution or access network. It has particular application in helping to identify the cause of the loss of connection to a powered remote node located at or near the customer end in the access network.
With global demands for ever-greater data transmission capacities and speeds, optical fibre has been increasingly deployed in preference to copper pairs to points nearer customers in the access network. One development is the “FTTx” point-to-multipoint network architecture, versions of which include Fibre to the Cabinet (FTTC) and Fibre to the Distribution Point (FTTdp) in which optical fibre is typically used between the local exchange and the named point in the access network. FTTx networks enable significantly higher numbers of end user customers to be served by partly or wholly by optical fibre, and comprise increased amount of plant deployed in the access network. Optical fibres and optical fibre equipment give rise to different operational, and test and diagnosis (T&D) issues from traditional copper cables, in which maintaining their physical condition is especially critical to obtaining acceptable performance levels. An example of an FTTx-based implementation is a Passive Optical Network (PON) wherein an Optical Line Terminal (OLT) or Network Termination Equipment (NTE) located in a local exchange at the head end communicates with nodes at the remote end in the access network, the nodes taking the form of Distribution Points (DPs or Distribution Point Units (DPUs)), Optical Network Terminals (ONTs), Optical Network Units (ONUs) and the like.
Active Remote Nodes (RNs) in the access network can be arranged to receive their power from the exchange end or a local power supply. Alternatively, power can be sent to the RN from the customer end e.g. using the Reverse Power Fed (RPF) method, especially those RNs such as DPs which are located very close, relatively speaking, to customer premises. For present purposes, an “RN” can be any node which is remote from the head end which is used to connect to customers, and typically has the function of adapting, amplifying or converting a signal to enable communication networks to be set up between the end user and the head end. An RN can draw power from one or more customer premises or source types (e.g. from a co-located power supply as well as from customer buildings). The numbers of RNs powered in this manner are anticipated to increase following recent developments such as G.Fast-based technology and network configurations in which the fibre is pushed further into the access network beyond the well-known FTTC street cabinet. G.Fast is a Digital Subscriber Line (DSL) ITU standard which promises greater data capacities and deliver improved transmission speeds.
When communications or connections between the RN and the exchange at the head end are lost, the network operator is typically obliged to take action to reinstate the connection e.g., under the terms of an agreed SLA, by removal of the cause of the problem. Such losses of connection can arise from a variety of causes. For present purposes, the causes can be considered according to whether they are (i) structural, or (ii) non-structural in nature. Structural causes include severance of the cable, loose connectors, broken splices or defects in the physical integrity of the optical cable or link (8) along the length between the RN and the head end which prevents or adversely affects the propagation of the light down the line. Structural cable conditions of this kind can be detected and located using an Optical Time Domain Reflectometer (OTDR), as described in e.g. EP2034635 and EP2670067, using methods which will be discussed further below. The effect on the signal or connection could range from the relatively mild (e.g. in the event of a macrobend in the fibre, or a loose connector so that some light can still be propagated down the line), to the fatal (e.g. when the cable is completely cut through by an excavator). At the head end, the affected link to the RN goes “dark” as the connection is lost. In such events, the network operator is usually obliged to take action to avoid or reduce the resulting downtime suffered by the customer(s).
Non-structural causes of connection loss with the RN include optical transceiver failure, component failure, or environmental factors e.g. excessively high temperature, or power outages. In the last case, all connection is lost when the RN loses power, so that optical signals cannot be received at the RN and the optical link to the head end at the local exchange goes dark. The RN could lose power for a number of reasons: it could be the result of equipment failure which would need action from the network operator. On the other hand, the outage could be due to a power failure, a different issue requiring a different course of action. The RN might have been deliberately powered down by the network operator e.g. for planned firmware or software upgrades. Yet another possibility is that the RN is turned off intentionally by customers, who control the power supply of a reverse powered RN. For example, the RN might be turned off overnight to conserve energy, or when the customer goes on holiday. It can therefore be understood that not all powered-down RNs need investigation or other action from the network operator as the loss of signal at the head end does not derive from operational problems in the network for which the network operator is responsible. This may be contrasted with structural causes of connection loss, which will almost always require network operator action and intervention.
It is known that different types of causes for loss of service or connection to customers can be remotely identified while the RN is up and active. For example, optical transceiver faults are indicated by alarms linked to the transmitter bias current; environmental alarms indicate excessively high operating temperatures; Ethernet layer alarms and alarms/diagnostics are associated with the copper line side DSL equipment. There are situations when the cause of loss of connection cannot be diagnosed by transmission of the above messages, such as when the link is physically broken, or unplugged from the RN, or when RN powers down. In a loss of power situation, the RN will not be able to generate such messages, and indeed it would not be likely to be able to receive interrogations from the head end in the first place. Without information about the cause of the loss of connection which would determine the kind of reparative action that should be taken (if any), it is uncertain if an engineer should be sent out to investigate. Depending on e.g. the QoS policy adopted by the network operator, an engineer may be sent out regardless; or else it may be decided to take no action until customer feedback (in the form of complaints) start arriving. Either option is at best a guess and unsatisfactory in the absence of intelligence about the cause of the power loss. Accordingly, there is a need to find out the cause of the connection loss, to make a cost-efficient decision about how to respond, preferably from the head end of the link.
A known technique for the remote monitoring of the power status of a RN from the exchange end include the generation of a “dying gasp” message (referenced in ITU-T Recommendation G.991.2 (December 2003) section 7.1.2.5.3) from the affected device at the customer end (such as the RN). The signal is typically sent to the digital subscriber line access multiplexer (DSLAM) when a power outage occurs. It can serve as an alert to e.g. the Network Management System (NMS) of a shortly-impending power source interruption if the RN is arranged so that a capacitor at the RN holds just enough power to send a last signal over the fibre link to the exchange before all power is lost. Alternatively, the RN can be provided to draw power for a brief period from another source so that the message can be sent. The transmission of the dying gasp message ends the session, and it is typically used to collect real-time and historical near and far-end link performance statistics. A dying gasp signal is however not always reliable as an indication of the cause of the loss of connection between the RN and the head end. This is because it may not be sent, may be corrupted, or may fail to be processed at the recovering end. The problem is compounded due to the impossibility of its retransmission after power is lost, and due to the ending of the session after it is sent. It is also noted that at present, the dying gasp signal is specified as being only optional in the GPON standard, while there is no specification for optical point to point networks at all. Furthermore, this approach does not allow for constant monitoring of the link, which rules out the possibility of a quick reaction to real time alerts of a potential fault condition.
While previous FTTx customers comprised enterprise and businesses in relatively small numbers, increasingly larger proportions of customers are residential entities. Not only are there are now considerably more access network nodes of all varieties which need cost-efficient management and maintenance, but residential customers will be significantly more price-sensitive than commercial entities. Provisioning current and future FTTx customers requires solutions which can cope with the volume of activity involved in the operation, testing and diagnosis of such networks in a cost-efficient manner. In particular, residential customers cannot and should not be ignored in an RN connection loss event just because they are paying lower rates for their service. With the numbers of remotely-powered RNs expected to grow at a rapid pace however, it would be highly inefficient if each case of a RN which has lost its connection had to be investigated without foreknowledge of the possible cause of the outage.
It would be desirable to remotely (preferably at the head end) determine if the cause of an unplanned loss of connection to a RN is an operational fault needing network operator action. Specifically, it would be desirable to have a reliable solution which goes toward discovering the cause of the connection loss without having to despatch an engineer to the location of the RN in the situation when the RN is unable to send and/or receive signals and which can no longer be managed. It has been realised with the current and imminent deployment of increased numbers of reverse powered RNs in particular, the chances have increased that a disconnection caused by loss of power does not require investigation as the powering down stems from customer choice. The technical implementation of such a solution in an optical network would help address the abovementioned problem.
According to the invention there is provided an optical network node capable of being powered, comprising                a transceiver,        a reflector arranged to reflect an optical signal, and        a switch arranged to direct the optical signal to the transceiver or the reflector in dependence on whether the optical network node is powered.        
An optical signal transmitted to an optical network node as configured according to the invention over an optical link, can be received by the node as along as the structure of the link to the node does not include physical discontinuities. The RN need not be powered on to receive the optical signal. A switch directs the signal received at the RN to a reflector in dependence on the power status of the RN. In one implementation, the switch directs the received optical signal to the reflector only when the RN is powered down. The reflector is arranged to generate a reflection of the optical signal: in contrast with the error messages and alarms of the prior art, the RN need not be powered up to generate and to send the reflected optical signal, which can serve as an alarm or such message back to the head end via the optical link to such other location as required. Embodiments of the invention can therefore very reliably indicate or confirm that the cause of a loss of connection event is down to the RN being powered down.
In preferred embodiments, the switch comprises a micro-electromechanical systems switch, while a variety of implementation options are available for the reflector such as a Fibre Bragg Grating (FBG) reflector; a thin film filter; an optical fibre having a cleaved end with a predetermined reflective value; an optical fibre having a cleaved end with a predetermined reflective pattern; and an optical fibre having a cleaved end of a predetermined length having a cleaved end. The skilled person would appreciate that it is possible to use more than one of the reflector designs in conjunction with each other.
Preferred applications of the invention can be used to detect causes of loss of connection other than a powered down RN. For example, an arrangement in which the connection socket (into which the optical connector of the optical fibre cable or link is received) includes a reflecting element, can help to detect or confirm that the loss of connection is due to the cable being unplugged from the RN.
According to another aspect of the invention there is provided a telecommunications network including an optical network node according to the invention operationally linked by an optical fibre to an optical transmitter, the switch being arranged to direct an optical signal output by the optical transmitter to the reflector in dependence on whether the optical network node is powered.
Preferably the optical signal output by an optical transmitter in the form of an OTDR, which can output pulsed or continuous signals to the RN. The optical transmitter is preferably located at the head end of the link between the RN and the OLT and/or network management components. Reflected signals output by the RN are preferably reflected back to the optical transmitter located at the head end, enabling a network operator to remotely monitor the link for specific causes of any connection loss events. As noted, the reflections can be directed as needed to any other location e.g. if the network operator's T&D activities are carried out at a central location and not necessarily at the local exchange where the relevant OLT is sited. Ideally, the monitoring activity is carried out continuously to enable real time or near-real time responses when problems arise.
In an optical network based on G.Fast technology, a network management system preferably includes a persistent management agent which can act as the RN's agent to receive messages and alarms concerning a loss of connection between the optical network node and the optical line terminal, when the RN itself is unable to do so. This relieves the network management system of the need to keep sending alarms to an unresponsive node which may not be fatally “dead”. It also allows for management instructions relating to reconfigurations and the like, to be held and delivered later to the RN if and when it re-activates.
Related to the abovementioned optical connector for plugging into the RN, the connector itself may be provided with its own reflector, which can generate a reflection advantageously giving an unambiguous indication that the fibre link to the RN is intact.
According to another aspect of the invention there is provided a method of remotely detecting the power status of an optical network node, comprising:                using a transceiver to send or receive an optical signal at the optical network node, and        in dependence on whether the optical network node is powered, using a switch to direct the optical signal to the transceiver or to a reflector arranged to reflect the optical signal.        
In a preferred application of the method, the optical signal serving as a test signal is transmitted and received at a wavelength which can be used at the same time during normal data transmissions, and sent continuously. In the method, the test optical signals are preferably compared against a base or reference value which represents the expected length or distance of the link between the two ends.